U.S. patent application number 10/524790 was filed with the patent office on 2006-06-22 for method for the production of isobutene from commercial methyl tert-butyl ether.
This patent application is currently assigned to OXENO OLEFINCHEMIE GMBH. Invention is credited to Rainer Malzkorn, Franz Nierlich, Udo Peters, Axel Tuchlenski.
Application Number | 20060135833 10/524790 |
Document ID | / |
Family ID | 31197203 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060135833 |
Kind Code |
A1 |
Malzkorn; Rainer ; et
al. |
June 22, 2006 |
Method for the production of isobutene from commercial methyl
tert-butyl ether
Abstract
The invention relates to a process for preparing high-purity
isobutene from industrial methyl tert-butyl ether (MTBE) and the
economical utilization of the secondary streams.
Inventors: |
Malzkorn; Rainer; (DORSTEN,
DE) ; Nierlich; Franz; (Marl, DE) ; Peters;
Udo; (Marl, DE) ; Tuchlenski; Axel; (Weinheim,
DE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
OXENO OLEFINCHEMIE GMBH
PAUL-BAUMANN-STRASSE 1
MARL
DE
45772
|
Family ID: |
31197203 |
Appl. No.: |
10/524790 |
Filed: |
July 11, 2003 |
PCT Filed: |
July 11, 2003 |
PCT NO: |
PCT/EP03/07543 |
371 Date: |
February 16, 2005 |
Current U.S.
Class: |
585/638 |
Current CPC
Class: |
Y02P 20/127 20151101;
Y02P 20/10 20151101; C07C 1/20 20130101; C07C 1/20 20130101; C07C
11/09 20130101 |
Class at
Publication: |
585/638 |
International
Class: |
C07C 1/00 20060101
C07C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2002 |
DE |
102 38 370.7 |
Claims
1. A process for preparing isobutene by acid-catalyzed dissociation
of methyl tert-butyl ether (MTBE), said process comprising:
fractionating a feed mixture comprising MTBE, C.sub.4- and
C.sub.5-hydrocarbons, methanol, methyl sec-butyl ether, TBA and
C.sub.4 oligomers to give a) a fraction a), comprising MTBE, MSBE,
TBA and C.sub.4 oligomers. and b) a fraction b), comprising
C.sub.4- and C.sub.5-hydrocarbons, MTBE and methanol, c)
dissociating the MTBE present in the fraction a) into methanol and
isobutene. and d) dissociating the dissociation product from c)
which comprises unreacted MTBE, methanol, isobutene and low boilers
and high boilers in a column into an isobutene-containing top
product and a bottom product comprising the unreacted MTBE and the
major part of the methanol from the dissociation, and recirculating
the bottom product to the feed mixture.
2. The process as claimed in claim 1, wherein the C.sub.4
oligomers, MSBE and TBA are separated off from the fraction a) by
means of a distillation, in which they are taken off as bottom
product.
3. The process as claimed in claim 1, wherein the C.sub.4
oligomers, MSBE and TBA are separated off from the fraction a) by
means of a bleed stream.
4. The process as claimed in claim 1, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is fractionated in a purification
column to give a bottom product consisting of pure isobutene and a
top product comprising isobutene and volatile by-products.
5. The process as claimed in claim 1, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is scrubbed with water, and
subsequently fractionated in a purification column to give a bottom
product consisting of pure isobutene and a top product comprising
isobutene and volatile by-products.
6. The process as claimed in claim 4, wherein water present in the
isobutene-containing stream is removed by means of a decanter.
7. The process as claimed in claim 4, wherein water present in the
isobutene-containing stream is removed by means of a decanter
located in the top section of the purification column.
8. The process as claimed in claim 4, wherein water present in the
isobutene-containing stream is removed by means of a decanter which
is located at a the side offtake of the purification column.
9. The process as claimed in claim 1, wherein the dissociation of
step c) and the separation of the isobutene in step d) from the
MTBE present in fraction a) are carried out in a reactive
distillation column.
10. The process as claimed in claim 2, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is fractionated in a purification
column to give a bottom product consisting of pure isobutene and a
top product comprising isobutene and volatile by-products.
11. The process as claimed in claim 3, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is-fractionated in a purification
column to give a bottom product consisting of pure isobutene and a
top product comprising isobutene and volatile by-products.
12. The process as claimed in claim 2, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is scrubbed with water, and
subsequently fractionated in a purification column to give a bottom
product consisting of pure isobutene and a top product comprising
isobutene and volatile by-products.
13. The process as claimed in claim 3, wherein the
isobutene-containing stream, which has been separated off from the
dissociation product from c), is scrubbed with water, and
subsequently fractionated in a purification column to give a bottom
product consisting of pure isobutene and a top product comprising
isobutene and volatile by-products.
14. The process as claimed in claim 5, wherein water present in the
isobutene-containing stream is removed by means of a decanter.
15. The process as claimed in claim 5, wherein water present in the
isobutene-containing stream is removed by means of a decanter
located in the top section of the purification column.
16. The process as claimed in claim 6, wherein the water present in
the isobutene-containing stream is removed by means of a decanter
located in the top section of the purification column.
17. The process as claimed in claim 5, wherein water present in the
isobutene-containing stream is removed by means of a decanter which
is located at a side offiake of the purification column.
18. The process as claimed in claim 6, wherein the water present in
the isobutene-containing stream is removed by means of a decanter
which is located at a side offtake of the purification column.
19. The process as claimed in claim 7, wherein the water present in
the isobutene-containing stream is removed by means of a decanter
which is located at a side offtake of the purification column.
20. The process as claimed in claim 2, wherein the dissociation of
step c) and the separation of the isobutene in step d) from the
MTBE present in fraction a) are carried out in a reactive
distillation column.
Description
[0001] The invention relates to a process for preparing high-purity
isobutene from industrial methyl tert-butyl ether (MTBE) and the
economical utilization of the secondary streams.
[0002] Isobutene is a starting material for the production of butyl
rubber, polyisobutylene, isobutene oligomers, branched
C.sub.5-aldehydes and C.sub.5-carboxylic acids. Furthermore, it is
used as alkylating agent and intermediate for the production of
peroxides.
[0003] Isobutene can be obtained by dehydrogenation of isobutane.
However, sufficiently large amounts of pure isobutane are not
available.
[0004] In industrial streams, for example, in the C.sub.4 fraction
from a steam cracker or an FCC unit, isobutene is present together
with saturated and unsaturated hydrocarbons. Owing to the small
boiling point difference or the very low separation factor between
isobutene and 1-butene, isobutene cannot be separated economically
from these mixtures by distillation.
[0005] The isobutene can be separated off from these C.sub.4
fractions in various ways, depending on which further olefins are
to be obtained. The first step which is common to all work-up
methods is removal of the major part of the butadiene and other
multiply unsaturated hydrocarbons. If butadiene can readily be
marketed or there is an in-house demand, it is separated off by
extraction or extractive distillation. Otherwise, it is selectively
hydrogenated to linear butenes down to a residual concentration of
about 2000 ppm by mass. In both cases, the stream which remains is
a hydrocarbon mixture (known as raffinate I or hydrogenated
cracking C.sub.4) which comprises the saturated hydrocarbons
n-butane and isobutane together with the olefins isobutene,
1-butene and 2-butenes (cis and trans).
[0006] If 2-butene or a mixture of linear butenes having a high
2-butene content is to be obtained in addition to isobutene, the
abovementioned mixture, which typically contains not more than 1%
of butadiene (C.sub.4 stream from FCC, raffinate I or hydrogenated
cracking C.sub.4), is hydrogenated and hydroisomerized, i.e.
residual butadiene still present is selectively hydrogenated down
to a residual content of below 5 ppm and 1-butene is isomerized to
the 2-butenes. The equilibrium ratio of 1-butene to the two
2-butenes is, for example, about 1/17, i.e. far on the side of the
2-butenes, at 80.degree. C. Owing to the small boiling point
differences, distillation of the hydroisomerization mixture gives
only a mixture of isobutene, 1-butene and isobutane as top product,
from which the isobutane can be separated off by distillation. The
bottom product obtained is an isobutene-free mixture comprising
2-butenes. Even if the hydroisomerization is carried out in a
reactive distillation column, no completely 1-butene-free isobutene
is obtained, as described, for example, in EP 0 922 018. This
isobutene is therefore not suitable for some applications.
[0007] Isobutene can be separated off from a C.sub.4-olefin mixture
via the steps selective conversion into a derivative, separation of
the derivative from the remaining hydrocarbon mixture and
dissociation of the derivative.
[0008] Isobutene can easily be converted into derivatives by means
of water or alcohols. The reaction of isobutene-containing streams
with water gives tert-butanol (TBA) which can easily be
redissociated into isobutene and water. The main disadvantage of
this separation process is the TBA synthesis, which, owing to the
low solubility of water in C.sub.4-hydrocarbons, gives only low
space-time yields.
[0009] The addition of methanol onto isobutene in
C.sub.4-hydrocarbon streams to form MTBE proceeds substantially
more quickly than the addition of water. Industrial MTBE is a
valued fuel component for four-stroke engines for the purpose of
increasing the octane number. Owing to the ease with which it can
be prepared and its large market volume, it is an inexpensive
precursor for pure isobutene.
[0010] For this reason, the industrial procedure is usually to
react an isobutene-containing C.sub.4 fraction with methanol to
form MTBE, purify the latter and redissociate it into isobutene and
methanol.
[0011] This process has the disadvantage that it is difficult to
prepare isobutene having a purity of greater than 99.9%. Industrial
MTBE (fuel grade) further comprises C.sub.4- and
C.sub.5-hydrocarbons, C.sub.4-oligomers (C.sub.8-,
C.sub.12-hydrocarbons), 2-methoxybutane (MSBE), methanol and TBA.
These materials and their downstream products and also other
by-products formed from MTBE during the dissociation can
contaminate the target product isobutene.
[0012] Integrated processes for preparing high-purity isobutene
from C.sub.4-streams via the preparation of MTBE and its
dissociation are known and described, for example, in U.S. Pat. No.
5,567,860. Here, isobutene-containing C.sub.4 streams are firstly
etherified with methanol to give, depending on the conversion, a
mixture of MTBE, MSBE, unreacted C.sub.4-hydrocarbons, methanol,
water, DME, C.sub.4 oligomers and also C.sub.3- and
C.sub.5-hydrocarbons as contaminants in the C.sub.4 stream. This
product mixture is fractionally distilled to give a low-boiling
fraction comprising the C.sub.3-, C.sub.4- and
C.sub.5-hydrocarbons, methanol and DME and a high-boiling fraction
comprising C.sub.4 oligomers. MTBE and MSBE are obtained in a side
stream taken from the column and are then passed to the
acid-catalyzed dissociation. The dissociation reaction accordingly
gives isobutene, n-butene and methanol as main constituents
together with unreacted MTBE and MSBE. This product mixture is in
turn purified by distillation, with the C.sub.4/methanol azeotrope
comprising isobutene and n-butene and DME being taken off as
low-boiling fraction. To obtain the target product, viz.
high-purity isobutene, this fraction has to be purified by means of
at least one water scrub and a distillation. The high-boiling
fraction obtained from the dissociation reaction (MTBE, methanol,
MSBE) is fractionated to give methanol as high boiler and an
azeotrope of methanol, MTBE and MSBE as low boiler. These fractions
are each recirculated to a point upstream of the etherification
stage or the dissociation stage.
[0013] This process is complicated in that the target product
isobutene has to be freed of the accompanying substances in the
C.sub.4 feed stream and unreacted reaction products or by-products
from the etherification and dissociation reactions in a plurality
of columns and scrubbing stages. Furthermore, an integrated process
should make it possible for unreacted materials such as MTBE or
isobutene-containing C.sub.4 streams to be separated off in a
simple fashion and be recirculated to the appropriate reaction
stages. In the ideal case, isobutene-containing C.sub.4
hydrocarbons and recovered methanol would be separated off at a
point in the process and reused for the preparation of MTBE. On the
other hand, unreacted MTBE should be obtained as a separate stream
and be recirculated to the ether dissociation reaction.
[0014] It is therefore an object of the present invention to
provide a process for preparing isobutene from MTBE which can be
operated using very few separation stages and with few recycle
streams.
[0015] The present invention accordingly provides a process for
preparing isobutene by acid-catalyzed dissociation of methyl
tert-butyl ether (MTBE), which comprises [0016] fractionating a
feed mixture comprising MTBE, C.sub.4- and C.sub.5-hydrocarbons,
methanol, methyl sec-butyl ether, TBA and C.sub.4 oligomers to give
[0017] a) a fraction a) comprising MTBE, MSBE, TBA and C.sub.4
oligomers and [0018] b) a fraction b) comprising C.sub.4- and
C.sub.5-hydrocarbons, MTBE and methanol, [0019] c) dissociating the
MTBE present in the fraction a) into methanol and isobutene and
[0020] d) separating off an isobutene-containing stream from the
dissociation product from c) and recirculating the remainder to the
feed mixture.
[0021] The MSBE present in the feed mixture passes unspecifically
into the two fractions a) and b), but is removed in an advantageous
fashion by means of a bleed stream from the fraction a).
[0022] The process of the invention can easily be linked to an
existing MTBE plant, so that the recycle streams of methanol and
C.sub.4-hydrocarbons can be reused for the preparation of MTBE. It
is also possible to use industrial MTBE of fuel quality or with
even lower specifications.
[0023] Compared to the prior art, the process of the invention
achieves a particularly elegant removal of impurities present in
the feed mixture and streams to be recirculated to other process
stages. Thus, methanol and low-boiling impurities in the feed
mixture are separated off in the first distillation stage prior to
the dissociation reaction. This makes it possible for the isobutene
obtained in the dissociation reaction to be separated off
efficiently, since interfering accompanying materials have already
been separated off. The recirculation of the methanol obtained in
the reaction to upstream of the dissociation reaction, or more
precisely upstream of the first distillation stage, results in
efficient circulation of the methanol with simultaneous removal of
by-products such as DME, C.sub.4 oligomers, TBA or MSBE in an MTBE
synthesis preceding the process of the invention.
[0024] A block diagram of a plant in which the process of the
invention can be carried out is shown in FIG. 1. MTBE (fuel grade)
(1) together with the bottom product (11) from column (9) is fed
into the column (2). A mixture of MTBE, methanol and C.sub.4- and
C.sub.5-hydrocarbons is taken off as top product (3). A part (6) of
the bottom product (4) from the column (2), which comprises
predominantly MTBE, is separated off to discharge high boilers
(TBA, diisobutene, MSBE). The other part (5) is fed to the
dissociation reactor (7). The reaction mixture (8) is fractionated
in the distillation column (9). The top product (10) obtained is an
isobutene which may further comprise methanol, dimethyl ether and
water. The optional work-up of this crude isobutene to give
high-purity isobutene is not shown in FIG. 1. The bottom product
(11) from the column (9), which comprises undissociated MTBE, part
of the methanol formed in the dissociation and high boilers, is
recirculated to the column (2). In place of the reactor (7) and the
column (9), it is also possible to use one or more reactive
distillation columns. All or part of the stream (3) can be conveyed
via line (12) to an optional etherification stage (13). Here, MTBE
is prepared from an isobutene-containing C.sub.4-hydrocarbon stream
(14), fresh methanol (15) and the recirculated methanol (12).
Stream (16) serves to bleed off unreacted components from the
isobutene-containing C.sub.4-hydrocarbon stream (e.g. n-butene and
aliphatic constituents).
[0025] The feed to the process of the invention can be industrial
MTBE of fuel grade. This typically comprises 98% by mass of MTBE
together with about 0.5% by mass of C.sub.4- to
C.sub.5-hydrocarbons, about 1% by mass of methanol, about 500 ppm
by mass of water and 2-methoxybutane. Preference is given to using
an industrial MTBE having a 2-methoxybutane content of less than
2500 ppm by mass, whose preparation is described, for example, in
DE 101 02 082.
[0026] It is also possible to use MTBE grades having a methanol
content significantly higher than 1% by mass, e.g. MTBE/methanol
mixtures having a ratio of 80:20, 90:10 or 95:5 can be processed
without problems. These mixtures can naturally further comprise the
accompanying materials mentioned above in an amount of .ltoreq.3%
by weight.
[0027] In the process of the invention, the C.sub.4- and
C.sub.5-hydrocarbons in the MTBE are removed by distillation
together with the MTBE-methanol minimum azeotrope. This gives a
distillate comprising MTBE, methanol and C.sub.4- and
C.sub.5-hydrocarbons. This mixture is advantageously fed to the
synthesis stage of an MTBE plant.
[0028] As a result of the recirculation of the bottom product (11)
from the distillation column (9), the C.sub.4- and
C.sub.5-hydrocarbons are removed from the MTBE feed in the column
(2) and the major part of the methanol formed in the MTBE
dissociation is also separated off in this column.
[0029] It has in practice been found to be useful for the
distillation column upstream of the dissocation reactor ((2) in
FIG. 1) to have from 10 to 60 theoretical plates, in particular
from 20 to 40 theoretical plates, of which from 10 to 30 are in the
enrichment section and from 10 to 30 are in the stripping
section.
[0030] The column is advantageously provided with internals such as
trays, random packing or ordered packing. The fractionation in this
column can be carried out at atmospheric pressure or under
superatmospheric pressure. Since the proportion of MTBE in the
MTBE/methanol azeotrope decreases with increasing pressure in the
pressure range from 1 to 25 bar and as little MTBE as possible
should be separated off together with the methanol, the
distillation is preferably carried out under superatmospheric
pressure, in particular in the pressure range from 5 to 25 bar,
very particularly preferably in the pressure range from 8 to 20
bar.
[0031] The reflux ratio in the column is from 1 to 10, in
particular from 2 to 7.
[0032] The bottom product from the column (2) upstream of the
dissociation reactor (step a) (FIG. 1) preferably has a content of
C.sub.4- and C.sub.5-hydrocarbons of less than 250 ppm by mass. It
contains a small amount of methanol and also TBA, 2-methoxybutane
and diisobutene as high boilers.
[0033] To separate off C.sub.4- and C.sub.5-hydrocarbons, their
oligomers and methanol from the dissociation together and at the
same time prevent accumulation of high boilers in the process, a
part (6) of the bottom product (4) from the column (2) can be bled
off continuously. The discharge of the C.sub.4 oligomers from the
fraction a) of the process of the invention can be carried out
either by means of a bleed stream or by means of a further
distillation stage, e.g. as bottom product. One possible use for
the bleed stream is work-up by distillation to produce high-purity
MTBE. For this it is necessary to reduce the methanol content of
the bottoms (4) from the column (2) to 50 ppm by mass, which can be
achieved by taking off an increased amount of distillate from the
column (2).
[0034] The dissociation of the bottom product, which comprises
predominantly MTBE, into isobutene and methanol can be carried out
over an acid catalyst located in a fixed bed.
[0035] One group of acid catalysts which can be used in the process
of the invention are solid ion exchange resins bearing sulfonic
acid groups.
[0036] Suitable ion exchange resins are, for example, those which
are prepared by sulfonation of phenol/aldehyde condensates or of
cooligomers of aromatic vinyl compounds. Examples of aromatic vinyl
compounds for preparing the cooligomers are: styrene, vinyltoluene,
vinylnaphthalene, vinylethylbenzene, methylstyrene,
vinylchlorobenzene, vinylxylene and divinylbenzene. In particular,
the cooligomers formed by reaction of styrene with divinylbenzene
are used as intermediate for the preparation of ion exchange resins
bearing sulfonic acid groups. The resins can be prepared in gel
form, macroporous form or sponge form. Strong acid resins of the
styrene-divinylbenzene type are sold, inter alia, under the
following trade names: Duolite C20, Duolite C26, Amberlyst A15,
Amberlyst A35, Amberlyst 36, Amberlite IR-120, Amberlite 200, Dowex
50, Lewatit K2431, Lewatit K2441, Lewatit K2621, Lewatit K2629,
Lewatit K2641.
[0037] The properties of these resins, in particular the specific
surface area, porosity, stability, swelling or shrinkage and
exchange capacity, can be varied by means of the production
process.
[0038] If desired, it is also possible to use commercial,
macroporous cation exchangers which have been modified by partial
ion exchange or by thermal desulfonation.
[0039] The dissociation of MTBE is carried out in one or more
reactors. When a plurality of reactors are used, these can be
connected in series or in parallel or both in series and in
parallel. it is possible to use various types of reactor, for
example fixed-bed reactors or shell-and-tube reactors or kettle
reactors.
[0040] The reactor(s) is/are operated isothermally, polytropically
or adiabatically, in a single pass or with an external recycle
loop.
[0041] The reaction temperature in the dissociation reactor in the
process of the invention is in the range from 60.degree. C. to
200.degree. C., preferably from 80.degree. C. to 120.degree. C.
When a plurality of reactors are used, the temperatures can be
identical or different.
[0042] The dissociation of MTBE can be carried out in the liquid
phase over acid ion exchange resins as described, for example, in
DE 3 509 292 or DE 3 610 704 or over acidic aluminum oxides as
disclosed, for example, in DD 240 739. In the latter case, the
reaction conditions (167.degree. C. and 1 bar or 297.degree. C. and
10 bar) are selected so that the dissociation of MTBE can also
proceed in the gas/liquid region. However, in the case of
dissociation processes carried out in the pure liquid phase, it has
to be noted that high MTBE conversions cannot be achieved in a
single pass because of the position of the thermodynamic
equilibrium. If pure MTBE is used in a dissociation reaction which
is preferably to proceed at 100.degree. C., an equilibrium
conversion of about 15 mol % is obtained on the basis of the
thermodynamics. A problem in the dissociation in the liquid phase
is the isobutene which is dissolved in the homogeneous liquid phase
and can undergo further reactions. The most important further
reactions of this type are acid-catalyzed dimerization and
oligomerization. For this reason, undesirable C.sub.8 and C.sub.12
components are found together with the desired target product
isobutene. The undesirable C.sub.8 molecules are
2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. High
reaction temperatures also favor the undesirable secondary reaction
of methanol to form dimethyl ether (DME). The formation of dimethyl
ether not only leads to a loss of methanol but also increases the
difficulty of purifying the isobutene.
[0043] The MTBE dissociation reaction can also be carried out in a
reaction distillation column, as disclosed in EP 0 302 336 or DE 4
322 712. EP 0 302 336 describes the elimination of methanol from
MTBE over an acid ion exchange resin which is located in the bottom
of the column. The dissociation of the ether in this case takes
place in the bottom of the column, i.e. the catalyst is continually
surrounded by a mixture of ether, olefin and alcohol. This is a
disadvantage for the preparation of isobutene, since higher
oligomers of isobutene are easily formed at relatively high
temperatures under the acidic conditions. In addition, the acid
centers of the catalyst are occupied by methanol, which leads to
undesirable formation of dimethyl ether. For this reason, a
different route is taken in DE 4 322 712. There, the tertiary ether
is fed into a reaction distillation column above the reaction zone,
and the enrichment section of the column serves for purifying the
isobutene while methanol is separated from the MTBE-methanol
azeotrope in the stripping section of the column. The azeotrope
goes back into the reaction zone. As acid catalyst, use is made of
a sulfated titanium dioxide extrudate. DE 100 20 943 discloses an
alternative process in which the ether to be dissociated (e.g.
MTBE) is introduced into a reactive distillation column below the
reaction zone. The actual dissociation takes place in an azeotrope
of the ether with the corresponding alcohol.
[0044] If the dissocation reactor (7) and the column (9) are
configured as a reactive distillation in the process of the
invention, preference is given to using structured catalytic
multipurpose packing as described, for example, in U.S. Pat. No. 5
348 710, EP 0 950 433, EP 0 428 265, EP 433 222. Such structured
packing which can be used for the purposes of the process of the
invention is, for example, commercially available as Katapak.RTM.
from Sulzer AG, Katamax.RTM. from Koch-Glitsch or Multipak.RTM.
from Montz GmbH. These types of packing are customarily constructed
of sheet metal, preferably mild steel, stainless steel, Hastelloy,
copper or aluminum, or structured sheets of mesh.
[0045] The dissociation mixture comprising unreacted MTBE,
methanol, isobutene, low boilers and high boilers is separated in a
column ((9) in FIG. 1) into an isobutene-containing top product and
a bottom product comprising the unreacted MTBE and the major part
of the methanol from the dissociation.
[0046] In a further process variant, it is also possible to
fractionate the bottom product from the column (9) (FIG. 1) in an
additional column (not shown in FIG. 1) to give an MTBE-rich bottom
product and a top product comprising mainly an MTBE/methanol
azeotrope. Part of this bottom product can be bled off to remove
high boilers. The other part is returned to the dissociation
stage.
[0047] If desired, the bottom product from column (9) or the
reactive distillation column can be fed to the synthesis stage of
an MTBE plant.
[0048] The isobutene which has been separated off from the reaction
mixture by distillation comprises methanol, water and dimethyl
ether. If desired, methanol is removed therefrom by extraction with
water using methods known per se.
[0049] The isobutene-containing stream can be fractionated in a
purification column to give a bottom product consisting of
high-purity isobutene and a top product comprising isobutene,
low-volatility by-products and possibly water. This purification
column can likewise be preceded by a water scrub to remove
methanol.
[0050] It is also possible to remove the water present in the
isobutene-containing stream which has been separated off (in
particular after a scrubbing stage) by means of a decanter. In the
decanter, a feed stream comprising isobutene, DME and water is
separated into a heavy, aqueous phase and a light, organic phase
comprising isobutene and DME.
[0051] In the process of the invention, this is preferably carried
out using a column provided with a decanter for the removal of
water located in the side stream from the colum. Locating the
decanter in the side stream minimizes the isobutene losses. It is
also possible to install the decanter at the top of the column.
[0052] FIG. 2 schematically shows such a procedure. The
isobutene-containing stream (10) obtained, for example, as shown in
FIG. 1 is scrubbed with water (16) in the extractor (15). This
isobutene stream (17) which further comprises dimethyl ether and
water is fed into the column (18) from which dimethyl ether is
taken off as top product (19) and high-purity isobutene is taken
off as bottom product (20). A liquid side stream (21) is taken off
below the feed point and is separated in the decanter (22) into an
aqueous phase (23) and an organic phase (24) which has been
depleted in water. Water (23) is taken off and the organic phase
(24) is recirculated to the column.
[0053] The pure isobutene column (18) preferably has from 25 to 50
theoretical plates, in particular 30-40 theoretical plates. The
isobutene to be purified is fed into the 15th to 30th theoretical
plate, in particular into the 18th to 24th theoretical plate, in
each case counted from the bottom. At a point located from two to
five theoretical plates below the feed point, the entire condensate
of this theoretical plate is taken off and passed to the decanter.
After the water has been separated off by mechanical means, the
organic phase is returned to the column at a point which is from
one to two theoretical plates further down.
[0054] In a particular embodiment of the water removal, the
decanter (22) is configured as a decantation tray within the
column, e.g. in the top section. In this case, only the aqueous
phase is obtained as side stream.
[0055] The distillation can be carried out at pressures of from 8
to 20 bar, in particular from 8 to 12 bar. The distillation
temperatures are dependent on the pressure. For example, the
temperature at the top at 9 bar is about 40.degree. C.
[0056] The isobutene obtained by the process of the invention has a
purity of from 99.90 to 99.98% by mass. in particular from 99.94 to
99.98% by mass, very particularly preferably from 99.96 to 99.98%
by mass.
[0057] In the process of the invention, internals comprising trays,
random packing or ordered packing can be used for the distillation
(column (2), (9) in FIG. 1; column (18) in FIG. 2). In the case of
column trays, the following types are used: trays having holes or
slits in the baseplate, trays having necks or chimneys covered by
bells, caps or hoods, trays having holes in the baseplate which are
covered by movable valves. It is also possible to use disordered
beds of various packing elements. They can be made of virtually all
materials, e.g. steel, stainless steel, copper, carbon, stoneware,
porcelain, glass, plastics, etc., and can have various shapes, e.g.
spheres, rings having smooth or profiled surfaces, rings having
internal webs or openings through the wall, wire mesh rings,
saddles and spirals. Packing having a regular geometry can be made
of, for example, metal sheets or meslhes. Examples of such packing
are Sulzer mesh packing BX made of metal or plastic, Sulzer
lamellar packing Mellapack made of sheet metal, structured packing
from Sulzer (Optiflow), Montz (BSH) and Kuhni (Rombopack).
[0058] The following examples illustrate the invention without
restricting its scope which is defined by the description and the
claims.
EXAMPLE 1
Dissociation of MTBE With Isobutene and Methanol Being Separated
Off
[0059] The dissociation of MTBE and the separation of the isobutene
produced and the methanol from the unreacted MTBE were carried out
in a plant as shown in FIG. 1 but without the MTBE synthesis stage
(13). The MTBE-methanol azeotrope and the C.sub.4- and
C.sub.5-hydrocarbon components were separated off using a column
(2) which was packed with Sulzer BX mesh packing and had 30
theoretical plates. The enrichment section had an internal diameter
of 50 mm and 15 theoretical plates and the stripping section had an
internal diameter of 80 mm and likewise 15 theoretical plates. The
isobutene was separated off in a column (9) having an internal
diameter of 50 mm which was likewise equipped with Sulzer BX mesh
packing and had 35 theoretical plates. The dissociation of MTBE was
carried out using a tube reactor (7) having an internal diameter of
21 mm and a length of 160 mm. As catalyst, use was made of a
commercial ion exchange resin from Bayer, Lewatit K2621. The tube
reactor was operated in a thermostated oil bath at 100.degree.
C.
[0060] The operating parameters of the two columns and the reactor
were as follows: TABLE-US-00001 Azeotrope Isobutene Dissociation
column column reactor (2) (9) (7) Pressure bar 10 5 Pressure bar 20
Temperature Temperature .degree. C. 100 Top .degree. C. 128 42 MTBE
conversion % 16 Feed .degree. C. 135 84 Bottom .degree. C. 148 106
Feed plate from the bottom 15 20 Reflux ratio kg/kg 4 3
[0061] The amounts and compositions of the individual streams are
shown in the following tables. industrial MTBE (Driveron.RTM.) was
used as feed. TABLE-US-00002 MTBE Column (2) Reactor Bleed Reactor
Column (9) feed distillate bottoms feed stream product distillate
bottoms (1) (3) (4) (5) (6) (8) (10) (11) Mass flow kg/h 8.0 3.6
22.5 20.2 2.2 20.2 2.1 18.1 Proportion by mass Dimethyl ether kg/kg
0.00000 0.00000 0.00000 0.00000 0.00000 0.00005 0.00048 0.00000
Isobutane kg/kg 0.00025 0.00055 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 Isobutene kg/kg 0.00000 0.00005 0.00000 0.00000
0.00000 0.10185 0.96873 0.00001 1-Butene kg/kg 0.00010 0.00022
0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 n-Butane kg/kg
0.00010 0.00022 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
trans-2-butene kg/kg 0.00025 0.00055 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 cis-2-butene kg/kg 0.00030 0.00066 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000 C5-hydrocarbons kg/kg
0.00200 0.00437 0.00007 0.00007 0.00007 0.00007 0.00000 0.00008
MTBE kg/kg 0.97840 0.67047 0.97346 0.97346 0.97346 0.81498 0.00000
0.91072 2-Methoxybutane kg/kg 0.00300 0.00284 0.00610 0.00610
0.00610 0.00610 0.00000 0.00682 Methanol kg/kg 0.00600 0.31706
0.00005 0.00005 0.00005 0.05759 0.03036 0.06079 tert-butanol kg/kg
0.00800 0.00000 0.00945 0.00945 0.00945 0.00733 0.00000 0.00820
Water kg/kg 0.00010 0.00295 0.00000 0.00000 0.00000 0.00053 0.00043
0.00054 Diisobutene kg/kg 0.00150 0.00005 0.01087 0.01087 0.01087
0.01150 0.00000 0.01285
EXAMPLE 2
Removal of DME and Water From the Isobutene Using a Decanter at the
Top of the Column
[0062] The purification of the isobutene by removal of dimethyl
ether and water was carried out as shown in FIG. 3 in a column
having a diameter of 50 mm. The column was equipped with Sulzer BX
mesh packing and had 35 theoretical plates. The decanter (22) from
which an aqueous phase (23) was taken off was located at the top of
the column (18). The feed to the plant came from an MTBE
dissociation (e.g. as shown in FIG. 1) with downstream removal of
methanol by extraction with water.
[0063] Operating parameters of the column: TABLE-US-00003 DME
column (18) Pressure bar 9 Temperature Top .degree. C. 57 Feed
.degree. C. 60 Bottoms .degree. C. 67 Feed plate from the bottom 20
Reflux ratio kg/kg 43
[0064] Flow data: TABLE-US-00004 Distillate Bottoms Stream taken
Rumback Feed from from Feed to off from from column column column
decanter decanter decanter (17) (19) (20) (21) (23) (24) Mass flow
Kg/h 6.000 0.199 5.798 8.736 0.002 8.733 Proportion by mass
Dimethyl ether kg/kg 0.01000 0.29995 0.00003 0.20378 0.04311
0.20382 Isobutene kg/kg 0.98930 0.69278 0.99987 0.78859 0.00226
0.78879 C5-hydrocarbons kg/kg 0.00010 0.00000 0.00010 0.00000
0.00000 0.00000 Methanol kg/kg 0.00001 0.00013 0.00000 0.00017
0.00190 0.00017 Water kg/kg 0.00060 0.00714 0.00000 0.00746 0.95273
0.00721
[0065] This experiment, in which the operation parameters had been
optimized in terms of isobutene purity and yield, showed that the
maximum dimethyl ether concentration in the distillate is limited
to about 30% by weight. As a result, about 2.5% of the isobutene is
lost via the distillate stream. This amount of loss cannot be
reduced using this decanter arrangement.
EXAMPLE 3
Removal of DME and Water From the Isobutene Using a Decanter
Located at the Side
[0066] These experiments were carried out as shown in FIG. 2 using
the same column as in Example 2, but the decanter was located below
the feed plate.
[0067] Operating parameters of the column: TABLE-US-00005 DME
column (18) Pressure bar 9 Temperature Top .degree. C. 41 Feed
.degree. C. 60 Bottom .degree. C. 67 Feed (17) to 21 side stream
(21) from 15 recycle (24) to 14 *from the bottom Reflux ratio kg/kg
170
[0068] Stream data: TABLE-US-00006 Distillate Bottoms Stream taken
Rumback Feed from from Feed to off from from column column column
decanter decanter decanter (17) (19) (20) (21) (23) (24) Mass flow
Kg/h 6.000 0.063 5.933 19.003 0.004 19.000 Proportion by mass
Dimethyl ether Kg/kg 0.01000 0.94949 0.00003 0.00556 0.00117
0.00556 Isobutene Kg/kg 0.98930 0.05051 0.99987 0.99329 0.00179
0.99348 C5-hydrocarbons Kg/kg 0.00010 0.00000 0.00010 0.00005
0.00000 0.00005 Methanol Kg/kg 0.00001 0.00000 0.00000 0.00031
0.00823 0.00031 Water Kg/kg 0.00060 0.00000 0.00000 0.00079 0.98881
0.00060
[0069] It can be seen from Example 3 that dimethyl ether
concentrations of greater than 95% by weight can be achieved in the
distillate by means of the particular arrangement of the decanter
and virtually no isobutene losses therefore occur. Thus, better
economics than in Example 2 are obtained.
* * * * *